The Occurrence of Platinum in Meteorites

Abstract

AbstractIn the course of the past twenty years, the writer has made a special study of the platinum content of meteorites. Practically complete analyses, including platinum, have been made of 29 falls, comprising 19 siderites, 9 aërolites, and one pallasite (siderolite). An extremely interesting fact brought out by these analyses is that platinum was present in every instance. Possibly of even more interest is the fact that the meteorites with high nickel content were found to contain much more platinum than those with low nickel content. Following are some representative assays: siderites containing about 16% of nickel yielded about 119 parts per million of platinum; siderites with 6% to 8% of nickel contained about 14 parts per million of platinum; while aërolites containing a little over 1% of nickel contained about 1.7 parts per million of platinum. While there are some exceptions to these ratios, the general uniformity is striking.Many analysts have reported the presence of small amounts of platinum in meteorites, but, on the other hand, it has sometimes been sought and reported as being absent. Possibly the reason for this fact is that the amount present was too small to be detected by the method used. The detection of platinum in extremely small amounts is a difficult and uncommon procedure, and few chemists have had an opportunity to gain the experience and skill necessary for this work. This fact can be appreciated readily when it is stated that the average amount of platinum found in these 29 falls amounts to only about 0.002%, which is less than the percentage of any other element determined.When platinum is mentioned in this article, the word is used collectively to include other metals of the platinum group, as iridium, rhodium, palladium, and perhaps others. The quantitative separation of minute quantities of these metals presents very great difficulties; therefore it was not attempted. Qualitative micro‐chemical tests for iridium were made on many of the samples, and iridium was found to be always present. Tests for palladium were made on 2 of the meteorites, with positive results. It may be that spectroscopic analysis, applied to the platinum metals obtained by the fire assay, would prove whether other platinum‐group metals were present.It is generally believed that all normal meteorites contain nickel and cobalt, and that the presence of these metals will help serve, in doubtful cases, to identify a meteorite. To these metals the writer would add platinum, which is apparently always present in meteorites in very small amounts.It is interesting to note that the close association of the platinum metals with iron, nickel, and cobalt in meteorites—and also in some terrestrial deposits—is paralleled by their close association in the periodic table of the chemical elements; here all of these elements occur grouped in a solid block.The method used by the writer for determining the presence of platinum in iron meteorites is a modification of the usual “fire” method. As the large amount of iron and nickel present would interfere, this is removed at the start by dissolving it in dilute sulfuric acid to which has been added 0.2 g. of copper sulfate. Copper precipitates on the iron and prevents loss of platinum. After the iron and nickel are dissolved, more copper sulfate is added, together with a little silver nitrate; the solution is saturated with hydrogen sulfide and later filtered. The filter paper is ignited, and the residue containing the platinum metals and silver is assayed by the fire method, using all the usual precautions. After cupeling the lead button, the bead containing the silver and platinum metals is parted by two treatments with 85% sulfuric acid, at a temperature of from 300° to 330° C. The residue of platinum metals is washed, dried, ignited gently, and weighed on a delicate gold balance. Most of the precautions needed in this work will be found described in Technical Paper No. 270, U. S. Bureau of Mines, entitled, “The Detection and Estimation of Platinum in Ores,” by C. W. Davis.